The present invention relates generally to bumpers, and more particularly, to energy absorbing vehicle bumper systems.
Bumpers typically extend widthwise across the front and rear of a vehicle and are mounted to rails that extend in a lengthwise direction. The bumper may also attach to a tow hitch assembly which, in turn, may be attached to the vehicle frame rails. Alternatively, the bumper may be attached to the tow hitch assembly which is then attached to the bumper assembly system before the bumper assembly system is attached to the vehicle. Energy absorbing bumper systems attempt to reduce vehicle damage as a result of a collision by managing impact energy and intrusion while not exceeding a rail load limit of the vehicle.
Steel beams are commonly used as bumpers. A steel beam is very stiff and provides structural strength and rigidity. Steel beams, however, are rather heavy. In addition, steel beams can crush or buckle and do not maintain a section modulus throughout an impact event.
Some bumper assemblies include shock absorbers. Such shock absorbers are positioned, for example, between a steel bumper beam and the vehicle rails. The shock absorbers are intended to absorb at least some of the energy resulting from an impact. Adding shock absorbers to a bumper assembly results in an added cost and complexity as compared to a steel beam. The shocks also add weight to the bumper assembly, which is also undesirable since such added weight may reduce the overall fuel efficiency of the vehicle.
Other known energy absorbing bumper systems include a beam and an energy absorber. The beam typically is steel. An energy absorber commonly used with steel beams is foam. Foam based energy absorbers typically have slow loading upon impact, which results in a high displacement. Further, foams are based effective to a sixty or seventy percent compression, and beyond that point, foams become incompressible so that the impact energy is not fully absorbed. The remaining impact energy is absorbed through deformation of the beam and/or vehicle structure.
The efficiency of a bumper system, or assembly, is defined as the amount of energy absorbed over distance, or the bumper system absorbs more energy over a shorter distance than a low energy absorber. High efficiency is achieved by building load quickly to just under the rail load limit and maintaining that load constant until the impact energy has been dissipated. Known shockless bumper systems that include a steel beam with a foam energy absorber generally have an efficiency less than fifty (50%).
Bumpers also have additional functions such as a horizontal step surface to support a customer's access to different areas of the vehicle that cannot be accessed from ground level.